This invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in methods and apparatus for optically reading magnetic information prerecorded on a magneto-optic recording element.
The method of optically reading magnetic information by use of the magneto-optic Kerr (or Faraday) effect is well known. Such method basically comprises the steps of scan irradiating a previously recorded magneto-optic media with linearly polarized radiation (e.g. emanating from a diode laser), and detecting small clockwise or counter-clockwise rotations, usually on the order of 1.degree. or less, in the plane of polarization of the reflected (or transmitted) beam. The direction of such rotation is determined by the vertical orientation (either up or down) of the irradiated magnetic domains representing the recorded information. If the linearly polarized radiation impinging on the disk is regarded as comprising the combination of two in-phase components, a left-hand circularly polarized (LCP) component and a right-hand circularly polarized (RCP) component, then the resulting Kerr rotation of the linearly polarized incident beam is understood as being caused by the media-induced difference in phase retardation between the LCP and RCP components.
As noted above, the amount of Kerr rotation produced by the magneto-optic media is relatively small (i.e. about .+-.1.degree.); as a result, the depth of modulation of the detector's output signal is correspondingly small. Various schemes have been proposed heretofore to enhance the signal-to-noise ratio (SNR) of the readout signal. One such scheme is disclosed in U.S. Pat. No. 4,571,650 issued in the names of Ojima et al.
Ojima et al. propose exploiting the so-called "self-coupling" effect between a readout laser beam and the magneto-optic media to control the instantaneous polarization mode, TE or TM, in which the laser operates. They note that the polarization mode of the laser can be switched between transverse electric (TE) and transverse magnetic (TM) modes by controlling the plane of polarization of the beam reflected by the media and returned to the laser medium. Ojima et al. propose using a Faraday rotator to set the plane of polarization of the reflected read beam at a certain critical angle (about which the TE/TM mode switching occurs) and using the small, plus or minus Kerr rotation angles produced by the recorded data to cause the laser to switch between its TE and TM modes. Being 90.degree. apart, the different polarization modes are readily distinguishable, making detection of the recorded information correspondingly simple.
While the idea of using the above "self-coupling" effect to detect oppositely oriented magnetic domains in a recording media is meritorious, the proposed implementation of this concept appears to be problematic. Because of the small Kerr rotation angles, very precise alignment of the Faraday rotator is required. Misalignment by 1.degree. would totally eliminate the effect on which the detection scheme is based. Moreover, since the critical angle (67.5.degree. in the disclosed embodiment) will vary from laser to laser, and may even change with usage and age, this detection scheme is not well-suited for use in a consumer product.